The external body of the shock is the same between MXR and EXR, but the valving is vastly different. The EXR has a 5.4kg/mm spring stock, and the MXR has a 6.0kg/mm spring stock. The shock springs are interchangeable. I put the stock 6.0 spring from my MXR on my EXR (since im running a 6.4kg/mm...